The present invention is generally directed to mechanical devices for making secure electrical connections to superconductive coils disposed within cryostats.
In the generation of images in medical magnetic resonance diagnostic systems, it is necessary to provide a temporally stable and spatially homogeneous magnetic field. The use of superconductive electrical materials maintained at a temperature below their critical transition temperatures provides an advantageous means to produce such a field. Main superconductive electrical coil windings are disposed in a vessel containing a cryogenic fluid such as liquid helium. Main coils and correction coils which provide the desired magnetic field uniformity are disposed in a cryostat which is essentially a thermal insulating device. The superconductive windings exhibit the particular advantage that electrical energy need not be supplied to the circuit once the desired current level is achieved in the main coils and the correction coils. However, electrical connections must be made to these interior coils at various intervals such as in the case of initial startup or after a quench condition in which the superconductive windings undergo a transition to the normal, that is, resistive state and the current flow is dissipated. In superconducting magnets, the main magnet coils can carry a current of 2,000 amperes while the correction coil currents are typically no more than 50 amperes.
While the correction coils and the main magnet coils typically comprise superconductive material, circuit energization is generally accomplished by means of normal (nonsuperconducting) conductors, which penetrate the nested set of vessels of the cryostat and significantly impair their insulating function or increase the rate of helium evaporation. It is desirable therefore to make and break electrical connections at superconducting temperatures, to minimize helium boiloff in steady state operation when the leads are retracted. Because of the extremely low temperatures (4.degree. K.) at which the connections are made and broken, there is a very strong tendency for frost to form on the electrical contacts. This frost typically includes both ice and solidified air. This frost, whether ice, air or both, can significantly impede the formation of a good electrical connection between the interior magnet circuit and an exterior energizing source. Electrical and mechanical properties of the contact surfaces must be adequate at the cryogenic temperatures since a high resistivity contact junction will generate heat and unnecessarily boil off cryogenic coolants.
A cam lever mechanism for a retractable lead system is described in U.S. Pat. No. 4,614,393 and assigned to the instant assignee. The cam lever mechanism tightens a split-ring terminal around the leads to exert high pressure at the contacts. One of the mating surfaces is serrated and preferably silver plated, and the other is coated with a thick layer of soft metal such as indium. Wear of the cam and linkages in the low temperature environment is a concern with regard to the life and reliability of the device especially when it is inaccessible for maintenance or repair.
Compact retractable cryogenic leads using a bayonet socket arrangement are described in U.S. Pat. No. 4,635,450, assigned to the same assignee as the present invention.
The retractable lead assembly comprises a tube defining a bayonet socket at one end and means for slidably and rotatably disposing the tube through the cryostat aperture. The tube extends inside and outside the cryostat with the bayonet end of the tube extending inside the cryostat. An electrically conductive rod is slidably disposed inside the tube. An electrically conductive terminal post with a transverse pin adapted to be engaged by the bayonet socket is mounted in the cryostat. Means external to the cryostat for rotating and slidably moving the tube to engage and disengage the socket with the terminal post are affixed to the tube. A threaded actuator external to the cryostat is coupled to the rod and the tube for adjusting the relative displacement between the rod and the tube, so that the rod can be urged against the post after the socket engages the pin. In operation, the threaded actuator can fail to apply a consistent load at the contact interface due to the presence of frost that accumulates at the warm end of the vapor cooled leads. This inconsistency can result in a high resistance contact or worse, the mechanical failure of inaccessible components.
It is an object of the present invention to provide a compact arrangement for high contact force electrical connections that can be operated by means of accessible, ambient-temperature, mechanical components.
It is another object of the present invention to provide high current leads that can make and break contact repeatedly and still attain a low resistivity connection.
It is a further object of the present invention to provide hydraulically actuated cryogenic leads which are easily disengageable and removable from a superconducting magnet as required.